1. Field of the Invention
The subject invention generally pertains to industrials doors and more specifically to a cable tensioner for such a door.
2. Description of Related Art
Sectional doors and rollup doors are two common examples of an industrial door. Sectional doors are often used as residential garage doors; however, they are also often used in warehouses and other industrial buildings. A sectional door typically includes a series of panels whose adjacent horizontal edges are connected by hinges. As the door opens or closes, the door panels travel along two lateral tracks. The tracks typically include a vertical section and an overhead section with a transitional curved section between the two. To close the door, the tracks guide the panels to a vertical position across the doorway. When the door opens, the hinges allow at least some of the panels to curve around onto the overhead section of the tracks. Such doors can be powered open and closed or moved manually.
To fit a sectional door underneath a standard 8-foot high ceiling of a typical residential garage, the vertical section of tracks is of limited height and the overhead section of tracks is generally horizontal. However, to take full advantage of generally higher ceilings in warehouses and other industrial buildings, a certain types of sectional doors known as a “high-lift” or “vertical lift” may be used. With a vertical-lift sectional door, the vertical section of tracks is extended and the overhead section may be nearly vertical or lie at an incline, such as a 15-degree incline from horizontal. The nearly vertical or inclined overhead section plus the extra vertical section provides greater clearance for material handling equipment, parts, and other equipment that may need to pass underneath the overhead tracks. With high-lift doors, one or more panels may store in the vertical and/or curved section of tracks when the door is at its fully open position.
A roll-up door typically includes a pliable roll-up panel or curtain that is wound about an overhead roller. Some roll-up panels are made of flexible fabric reinforced with several vertically spaced horizontal stays or wind bars. Other roll-up panels comprise a series of narrow, relatively rigid metal bars or segments that extend horizontally across the doorway. The segments are pivotally interconnected along their horizontal edges, so that the panel can wrap around the overhead roller.
To close a roll-up door, the roller pays out the panel as two vertical tracks along either side edge of the doorway guide the side edges of the panel generally along a vertical plane across the doorway. The rotation of the roller is reversed to open the door. Roll-up doors are typically either powered open and closed, or are powered open and allowed to close by gravity.
To ease the operation of a vertically moving door, a counterbalance system is often used to counteract the weight of the door panel or panels. Counterbalance systems typically include one or more deadweights or a spring. When deadweights are used, they are usually suspended from a cable that is wrapped about a drum fixed to an overhead rotatable shaft. Another drum fixed to that same shaft holds another cable that carries the weight of the door panels. The cable attached to the door panels and the cable attached to the deadweights are wrapped about their respective drums in opposite directions to create two opposing torques that are applied to the overhead shaft. With the two torques acting in opposite directions, the weights of the door panels and the deadweights generally cancel each other, which makes it easier to open and close the door. Doors with a counterbalance system employing a torsion spring operate in a similar manner; however, the torsion spring applies a torque to the overhead shaft that replaces the torque otherwise created by the cable-suspended deadweight.
To keep a cable neatly wrapped about its drum and to prevent the cable from getting tangled by overlapping itself, a drum may include a cable groove that runs helically around the drum. The groove creates a helical track that helps guide the cable in a similar helical pattern as the cable wraps and unwraps about the drum. This works well as long as the cable is maintained in sufficient tension. Under certain circumstances, however, the tension in a cable may be momentarily released, which can create sufficient slack in the cable to allow the cable to “jump a groove” and get tangled with itself on the drum. This can damage a cable, cause a door to jerk unexpectedly, or even prevent operation of the door until maintenance personnel corrects the problem. In severe cases, the cable may even jump completely off the drum.
There are many situations, some of which may be unknown, which can cause a momentary release of cable tension. When opening a door, for example, momentum may carry the door panel beyond its normal open position. This may momentarily unload the door's counterbalance system by lowering a deadweight onto the floor or by releasing the preload on a counterbalance's torsion spring. Or if a door is slammed shut, the door panel may come to an abrupt stop upon hitting the floor. Meanwhile, the angular momentum of the overhead shaft may allow the drum that supports the door panel to continue releasing cable. At the same time, the upward momentum of a deadweight may allow the deadweight to continue traveling upward, which could release the tension in the deadweight's cable as well. A cable can also become slack if whatever it is carrying (door panel or deadweight) gets snagged or caught upon traveling downward. In general, the cable that supports either the door panel or a deadweight may become slack and jump a groove whenever the relative speed or position of the deadweight and the door panel is mismatched.
These slack conditions typically occur from the deadweight hitting a ground surface during the raising of the door panel. For conditions where the door is rapidly raised, the door panel may overshoot its raised position, after the deadweight has hit the ground. Depending on the size and weight of the door, and the rapidity of the ascent, the door panel may rapidly freefall, imparting a high pulling force (i.e., a shock) on the cable. This shock condition can cause great strain on the cable, especially on cables that are quickly jerked from a slack condition to fully taught, by this downward freefall. Shock also occurs on cables traveling in the reverse direction, i.e., when the door is lowered. The deadweight and may overshoot it's fully raised position, when a rapidly descending door hits the ground. The deadweight may freefall from this overshoot condition and impart a shock force on the cable.